Using the Whiplash API you can create an application to do any task you would normally do on WhiplashMerch.com. For example, you could use the API to get a list of all orders that have been shipped recently, change order addresses before they've gone out, create new orders and more.
The Whiplash API is implemented as JSON or XML over HTTP using all four verbs (GET/POST/PUT/DELETE). Each resource, like Order, Item, etc., has its own URL and is manipulated in isolation. An authentification token is required for all calls, which is available on your preferences page.
We provide a testing server at http://testing.whiplashmerch.com, as well as a shared Sandbox customer account with an authentification key of Hc2BHTn3bcrwyPooyYTP
Let's take a look at the calls your application can make and the attributes that come back.